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Pebble-sized sandstone concretions, cemented by iron and manganese oxides, are found in several semi-arid regions in the world. Although recognized for a long time, their formation mechanisms have not yet been constrained from scientific studies. We have made extensive studies of the chemical composition, cosmogenic 10 Be and radiogenic U/Th concentrations, and biological fossils in ironstones from six sites in southern California. The ironstones exhibit appreciable enrichments of Mn, Zn, Mg, Ti, Fe, U, Th, and fossil bacteria. In addition to elemental data, we report here first observations of radionuclides, U, Th, K, and concentrations of cosmogenic 10 Be which we show are an excellent indicator of precipitation amounts. Our data favor the model that the ironstones formed within sandy beach ridges during wetter climates following dry climates during which aeolian sediment was added to the beach ridges. Iron, Mn, Zn and other trace element-rich leachates from the dust layers nurtured and accelerated bacterial activity in the beach ridges down to depths of a few meters, as first suggested by Abbott (1981). Our observations of trace-element enrichments and bacterial fossils support the model proposed by Abbott and underscore the fact that the ironstones are principally a product of bacterial activity, which concentrates the leachates in a narrow layer within the beach ridges. The extreme alternating dry/wet climatic conditions which existed in the past in southern California led to the formation of ironstone concretions within the ancient beach ridges, which provided suitable host mineralogy for their formation. The time periods represented by the ironstones from the six sites presumably cover the past ∼ 1 my. The recent surface explorations on the surface of Mars by the rovers SPIRIT and OPPORTUNITY (Squyres et al., 2006), showed that similar to southern California, extreme climatic conditions existed on Mars in its early history. It therefore seems that studies of ironstones may provide useful clues to the evolution of soils on Mars in such extreme climates, and possibly even provide criteria to evaluate whether any bacterial activity was present at that time. |
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